Spoon off all but about 3 tablespoons of the chicken fat from the pan. Be careful not to remove the fond and juices (the fat is clear - the juices are opaque).
Place the skillet over medium heat. Add the onions. Cook until they start to brown. This should take around 5-7 minutes.
Add the garlic ginger paste. Fry, stirring constantly until it stops sputtering. That usually takes about a minute.
Reduce the heat to medium low. Add the fenugreek seed and stir. Cook about 30 seconds.
Add the 3 tsp (yes I know - that's one tablespoon) tandoor masala, 1 and 1/2 tsp of kashmiri chili powder and 1/2 tsp of kosher salt.
Look at your pan. You will need some oil to fry the spices. A dry pan is a dangerous pan at this point. If it looks dry add a bit more oil. Toss in the tandoori masala/kashmiri chili/ salt mix, stir to thoroughly combine the spices and oil and cook, stirring constantly, for about 30-45 seconds.
Add the tomatoes and stir. Cook the resulting tomato, onion masala until the tomatoes start to break down. This should take about 5 minutes.
Add the coconut milk. Bring to a lively simmer and cook for 3 minutes.
Add the chopped baby spinach. Stir into the sauce and cook until the spinach is fully wilted.
Return the potatoes to the pot and simmer until the potatoes are warmed through.
To serve nap the plate or large bowl with a little sauce, place a few potatoes in the middle of plate and top with chicken.